Content displayed on a web page can be generated by one or more content item servers in response to content item requests that are generated during the rendering of the web page by a client device. The content item requests can be generated synchronously with respect to the rendering of the web page. Likewise, the content items received in response to the content item requests can be processed synchronously with respect to the rendering of the web page. For example, when a web page is rendered, a javascript may execute and request an advertisement from an advertisement server. If the advertisement is retrieved synchronously, the rendering of the web page is delayed until a requested advertisement is received from the advertisement server. Once the advertisement is received and rendered, e.g., displayed on the web page, the rendering of the remainder of the web page resumes.
A drawback of synchronous content item retrieval, however, is that if a content item server is slow or temporarily inoperable, then the remainder of the web page will not render. To mitigate the potentially deleterious effects of synchronous content item processing, an asynchronous content item environment can be defined, e.g., an iFrame. However, a web page publisher may desire that a content item be an integrated feature of a web page and not defined within an iFrame, or that a content item be presented in a synchronous content item environment. Additionally, it is often a relatively complex task to recode a web page source file to ensure that content items are retrieved asynchronously, e.g., recoding to ensure that content items are rendered in an iFrame.